Yellow fever virus (YFV) was at one time endemic in the United States and represents an emerging/re-emerging human pathogen that causes up to 20% mortality. Although YFV is typically spread by the bite of Aedes aegypti mosquitoes, it also represents a viral encephalitide with potential for use as a bioterrorism agent because;1) clinical isolates of YFV are available and/or easily obtained during natural outbreaks, 2) YFV grows to high titers in vitro with relatively simple equipment, and 3) YFV can cause lethal encephalitis when administered by an aerosol route. The current live attenuated YFV vaccine was developed in 1936 and following the development of a virus seed lot system, it has not been modified or otherwise improved in over 50 years. This vaccine causes 4 cases of encephalitis per million doses administered. The overall mortality rate following YFV vaccination is estimated at 1 to 2 deaths per million doses, indicating that this vaccine is as dangerous as the live smallpox vaccine, Dryvax. YFV vaccination of infants <9 months of age has been contraindicated since the 1960's due to the discovery of high rates of vaccine-associated encephalitis in this age group. More recently, YFV vaccination has been found to cause severe viscerotropic disease in a substantial number of patients >60 years of age (incidence rate is approximately 1:50,000 doses administered) and these cases result in approximately 50% mortality. This indicates that YFV vaccination is not only contraindicated in infants, but may soon be contraindicated in the elderly as well due to the increased risk of severe and life-threatening disease. Increased monitoring efforts have also documented several cases of vaccine-related fatalities in young, otherwise healthy adults with no known pre-existing immune deficiencies. To date, there is no alternative to live YFV vaccination. In this proposal, we will use a proprietary new technology to develop an inactivated vaccine formulation that can be used to immunize vulnerable populations such as infants and elderly, in addition to other healthy populations. Our preliminary studies demonstrate that this vaccine approach is feasible and highly immunogenic. In this project, we will evaluate candidate vaccine formulations, perform scale-up development, and test in vivo efficacy against lethal YFV infection. PUBLIC HEALTH RELEVANCE: In this feasibility Phase I proposal, we provide strong preliminary data demonstrating the antigenicity, immunogenicity, and protective efficacy of a proprietary new vaccine platform that can be used to develop a safer and more effective YFV vaccine.